In the turbine section of a turbine engine, the turbine rotor is circumscribed by a shroud such that the shroud is adjacent the tips of the rotor blades extending from the hub of the rotor. The shroud serves to channel the combustion gases through the turbine section of the turbine engine and prevents the bulk of the turbine engine's combustion gases from bypassing the turbine rotor blades. However, a portion of the gases are able to bypass the rotor blades through a gap present between the rotor blade tips and the shroud. Because the energy of the gases directed through the rotor blades is used to rotate the turbine rotor assembly and any compressor upstream of the turbine section, turbine engine efficiency can be increased by limiting the gases which are able to bypass the rotor blades through this gap.
Manufacturing tolerances, differing rates of thermal expansion and dynamic effects limit the extent to which this gap can be reduced. Any rubbing contact between the rotor blade tips and the shroud will spall the tips of the rotors. Spalling will tend to further increase the gap described above, thereby reducing engine efficiency. In addition, spalling tends to promotes structural fatigue in the rotor blades, causing the useful life of the rotor to be shortened.
As an alternative, it is well known in the art to form a dynamic seal between the rotor blades and the shroud by forming an abrasive tip cap on the end of one or more rotor blades, and more preferably, on each rotor blade. During operation of the turbine, the abrasive tip caps abrade a groove in the shroud as a result of numerous "rub encounters" between the abrasive tip caps and the shroud. The groove, in cooperation with the rotor blade tips as they partially extend into the groove, forms a virtual seal between the rotor blade tips and the shroud. The seal reduces the amount of gases which can bypass the rotor blades, and thereby improves the efficiency of the turbine engine.
Various materials and processes have been suggested to provide a suitable abrasive tip cap on turbine rotor blades. Typical abrasive materials used include silicon carbide, aluminum oxide, tantalum carbide and cubic boron nitride. Aluminum oxide, or alumina, is generally preferred because of its high temperature capabilities and oxidation resistance. In that such abrasive particles do not provide a structurally sound material, they are incorporated with a metal matrix, including for example, nickel or cobalt-base alloys, to provide a sufficiently strong structure which can be bonded to the blade tip. However, the thickness of such a metal matrix is often limited because of the structural weakness of the abrasive composition.
In some applications, it is conventional to apply the abrasive composition to the rotor blade tip using a thermal spray technique, such as plasma spraying or detonation gun spraying. While suitable for many purposes, thermal spray techniques are inefficient in that only part of the abrasive composition contacts and adheres to the rotor blade tip, while much of the thermal spray completely misses the target. More importantly, thermal spraying damages or destroys the morphology of the abrasive particles, making them unsuitable for the intended purpose. In addition, subsequent processes are typically necessary to provide the adhesion and structural integrity necessary for the abrasive composition to survive the hostile environment of a turbine engine. Such steps often include adhering the abrasive composition to the blade tip during a first heating and cooling cycle, and later depositing an additional quantity of the metal matrix over the abrasive composition through a second heating and cooling cycle, such as during hot isostatic pressing. As an alternative, it has also been suggested to melt the tip of the blade, such as with lasers, introduce the abrasive to the blade tip, and then resolidify the blade tip.
While the above processes may be suitable for some turbine blade structures, turbine blades used in modern gas turbine engines are often fabricated from cast high temperature nickel-base superalloys having a single crystal microstructure. Single crystal blades are characterized by extremely high oxidation resistance and mechanical strength at elevated temperatures, which are necessary for the performance requirements of modern turbine engines. However, the single crystal microstructure must not be affected by the process by which the rotor blade abrasive tip caps are secured to the rotor blades. In particular, the process must not recrystallize the single crystal microstructure of the rotor blade, such that the high temperature properties of the rotor blade are lost or diminished. As a result, processes which entail melting the rotor blade tip to the single crystal rotor blade are entirely unacceptable. In addition, repeated thermal cycling of the rotor blades runs the risk of degrading the single crystal microstructure of the rotor blade.
Thus, it would be desirable to provide an abrasive composition which can be readily formed into an abrasive blade tip cap and which can be attached to a turbine rotor blade in a single heating and cooling cycle so as to minimize any degradation of the microstructure of a single crystal turbine rotor blade.